Isolated gas sensor configuration

ABSTRACT

A gas sensor assembly detects a constituent in a gaseous stream. The assembly comprises: (a) a mounting surface, (b) a sensor mounted on the mounting surface and sensor capable of generating a detectable signal in the presence of the constituent, and (c) an enclosing structure. The enclosing structure comprises: (i) a walled component having a pair of vertically spaced ends and mounted at one end on the mounting surface so as to circumscribe the sensor, and (ii) a gas-permeable membrane attached at the other end of the walled component, thereby defining an interior volume within said enclosing structure. Flowing the gaseous stream across the membrane infuses a portion of the gaseous stream into said interior volume containing the sensor.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is related to and claims priority benefits from U.S.Provisional Patent Application Ser. No. 60/540,018, filed on Jan. 27,2004. The '018 provisional application is hereby incorporated byreference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to sensors for detecting the presence of aconstituent in a gaseous stream. More particularly, the presentinvention relates to a hydrogen gas sensor configuration in which thesensor is isolated within an enclosing structure that is interposedbetween a mounting surface and a gas-permeable membrane.

BACKGROUND OF THE INVENTION

In certain gas sensor applications, it is desirable to keep the sensorisolated from the external environment without impeding itsfunctionality. Such isolation can be for the purpose of reducing orminimizing heat loss, reducing or minimizing the amount of lightreaching the sensor, and/or reducing or minimizing the consequences ofmechanical intrusion. Often, a sensor is operated at a giventemperature, typically greater than that of the surrounding gas streamit is sensing. This is sometimes accomplished by the use ofheat-producing devices disposed on the same substrate as the gas-sensingdevice. When this is the case, there is a finite amount of heat lost tothe gas stream and structures surrounding the sensor. This heat loss isproportional to an amount of power loss from the entire system in whichthe sensor has been incorporated, and it is therefore desirable toreduce or minimize such heat loss. In addition, for some sensors it isdesirable to limit the amount of light reaching the sensors. For mostsensors, it is also desirable to limit mechanical or physical intrusion,either from particles entrained in the gas stream to be sensed oraccidental occurrences such as the device being dropped.

Conventional, prior art thermal isolation techniques include fabricatingthe sensor itself in such a way as to create structures to providethermal isolation (see, for example, U.S. Pat. Nos. 5,211,053,5,464,966, 5,659,127, 5,883,009 and 6,202,467). Such exemplary thermalisolation techniques were designed specifically for the type ofconstruction of the sensor involved and did not overcome the problemsassociated with heat loss at an assembly level, that is, where thesensor is configured as part of a greater assembly. Priorimplementations of such gas-sensing devices, such as catalytically-basedgas sensors, have employed different techniques to thermally isolate thedevice, such as suspending the device, within the gas stream beingsensed, using individual wires that electrically connect the sensingdevice to its downstream processing and control circuitry (see, forexample, U.S. Pat. No. 5,902,556), but these methods are not preferredfor a sensor with multiple connections.

The foregoing prior art solutions have the disadvantage of beingconsiderably more voluminous and bulky than is desirable for mostend-uses. Additionally, design parameters of the prior art sensors havetrade-offs, such as response-to-size and isolation-to-flow effects.“Response-to-size” refers to the relationship between the magnitude ofthe isolated gas volume exposed to a sensor and the amount of time for acomplete exchange of the gaseous constituents within the isolatedvolume; the smaller the size of the isolated gas volume, the more rapida complete exchange will occur. “Isolation-to-flow” refers to therelationship between the flow rate of a gas stream exposed to a sensorand the magnitude and rate of heat loss from the sensor; higher gasstream flow rates draw greater amounts of heat from a sensor morerapidly, thereby increasing power consumption by the sensor in order torestore lost heat. Moreover, such prior art solutions did not addressthe optical sensitivity of certain sensors. In this regard,capacitor-based devices are generally sensitive to light and cangenerate erroneous, stray signals upon exposure to light.

SUMMARY OF THE INVENTION

The present gas sensor assembly having a configuration in which thesensor is isolated overcomes one or more of the foregoing shortcomingsof prior art gas sensors. In particular, the present isolated gas sensorconfiguration reduces power consumption, limits the unfavorable effectsof ambient light, and limits mechanical intrusion through the use of ageometry that provides a gas-filled gap and an enclosing structure thatincludes a gas-permeable membrane.

In one embodiment, the present gas sensor assembly for detecting aconstituent in a gaseous stream comprises:

-   (a) a mounting surface;-   (b) a sensor mounted on the mounting surface, the sensor capable of    generating a detectable signal in the presence of the constituent    and;-   (c) an enclosing structure comprising:    -   (i) a walled component having a pair of vertically spaced ends,        the walled component mounted at one end on the mounting surface        and circumscribing the sensor; and    -   (ii) a gas-permeable membrane attached at the other end of the        walled component, thereby defining an interior volume within the        enclosing structure.        Flowing the gaseous stream across the gas-permeable membrane        infuses a portion of the gaseous stream into the interior volume        containing the sensor.

In a preferred embodiment of the present gas sensor assembly, thegas-permeable membrane has optical properties that inhibit passage oflight into the interior volume. The gas-permeable membrane isnon-transparent and, more preferably, opaque.

The gas-permeable membrane is preferably and, more preferably, apolytetrafluoroethylene-based membrane material.

In a preferred embodiment of the present gas sensor assembly, themounting surface is formed on a flexible circuit and the mountingsurface is planar.

In an illustrative embodiment of the present gas sensor assembly, thesensor is sensitive to hydrogen and is catalytically activated.

In a preferred embodiment of the present gas sensor assembly, the walledcomponent is tubular and is formed from a polymeric electricallyinsulative material, preferably an acetal resin.

A method of isolating, in a gas sensor assembly, a sensor capable ofgenerating a detectable signal in the presence of a gas streamconstituent, comprises:

-   (a) mounting the sensor on a mounting surface and;-   (b) enclosing the sensor in an enclosing structure comprising:    -   (i) a walled component having a pair of vertically spaced ends,        the walled component mounted at one end on the mounting surface        and circumscribing the sensor; and    -   (ii) a gas-permeable membrane attached at the other end of the        walled component, thereby defining an interior volume within the        enclosing structure;:        Flowing the gaseous stream across the gas-permeable membrane        infuses a portion of the gaseous stream into the interior volume        containing the sensor.

In a preferred embodiment of the isolating method, the gas-permeablemembrane has optical properties that inhibit the passage of light intothe interior volume, and the gas-permeable membrane is preferablynon-transparent.

A method of detecting a constituent in a gaseous stream, the methodcomprises:

-   (a) mounting a sensor on a mounting surface, the sensor capable of    generating a detectable signal in the presence of the constituent;-   (b) enclosing the sensor in an enclosing structure comprising:    -   (i) a walled component having a pair of vertically spaced ends,        the walled component mounted at one end on the mounting surface        and circumscribing the sensor; and    -   (ii) a gas-permeable membrane attached at the other end of the        walled component, thereby defining an interior volume within the        enclosing structure;-   (d) flowing the gaseous stream across the gas-permeable membrane    whereby a portion of the gaseous stream infuses into the interior    volume and the impinges upon the sensor; and-   (e) generating a detectable signal from the sensor in the presence    of the constituent.

In a preferred embodiment of the isolating method, the gas-permeablemembrane has optical properties that inhibit the passage of light intothe interior volume, and the gas-permeable membrane is preferablynon-transparent.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a schematic diagram showing a cross-sectional view of a firstbasic configuration of the present isolated gas sensor assembly.

FIG. 2 is a perspective view of one embodiment of a gas sensor assemblythat implements the configuration illustrated schematically in FIG. 1.

FIG. 3 is an exploded perspective view of the gas sensor assembly ofFIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

A first basic configuration of the present isolated gas sensor assembly10 is shown schematically in FIG. 1. Gas sensor assembly 10 includes aflexible circuit surface 15 upon which are mounted a sensor 22, anenclosing structure 30 that includes a walled component 32, and agas-permeable membrane 34, as depicted. An interior volume or gas-filledgap is formed within enclosing structure 30, bounded on the bottom byflex circuit 15 and sensor 22, bounded on the side by the walls ofcomponent 32, and bounded on the top by membrane 34.

In the depicted assembly, flex circuit surface 15 has a flexible (andfoldable) planar surface on which sensor 22 and enclosing structure 30can be securely mounted.

The mounting surface could conceivably assume surface configurationsother than planar depending upon the application in which the gas sensorconfiguration is to be incorporated. Such mounting surfaceconfigurations could include, for example, arcuate or spheroidalconfigurations in which the surface on which the sensor and enclosingstructure are mounted are non-planar.

Sensor 22 is capable of generating a detectable signal in the presenceof the gaseous constituent to be detected, which is hydrogen in theembodiments illustrated herein. The electrical signal generated bysensor 22 is conducted via copper traces (not shown) formed in the flexcircuit and transmitted to downstream electrical signal processing andcontrol circuitry that transforms the signal to a perceivable outputthat is indicative of the presence and concentration of the gas streamconstituent to be detected (hydrogen in the illustrated embodiment).

In the case of the hydrogen gas sensor specifically described herein,the sensor generates a signal derived from the catalytic reaction of thesensor components or elements with hydrogen that is present in the gasstream to be assessed. Suitable hydrogen sensor configurations includewide range sensors developed at Sandia National Laboratories (see R.Thomas and R. Hughes, “Sensors for Detecting Molecular Hydrogen Based onPD Metal Alloys”, J. Electrochem. Soc., Vol. 144, No. 9, September 1997;and U.S. Pat. No. 5,279,795), as well as the integrated sensorconfigurations developed by H2Scan LLC and described in applicationsfiled concurrently herewith.

Reducing or minimizing the heat loss through convective currents isenabled or facilitated by the cooperation of gas-permeable membrane 34with walled component 32 and flex circuit surface 15 to enclose gassensor 22 within enclosing structure 30. Since the function of a gassensor is to detect and measure the specific concentration of gaseousconstituent(s) in a given atmosphere, gas sensor assembly 22 isnecessarily exposed to some larger volume of a flowing gas stream (notshown). This gas stream can be subjected to currents that are eithernatural in occurrence or induced by the flow of the gas stream through aconduit (not shown). The isolation of sensor 22 from these currents byuse of the surrounding enclosing structure 30, including gas-permeablemembrane 34, provides a further reduction in power loss from sensor 22.

Gas-permeable membrane 34 allows the gas being sensed to reach thesensor 22 for the purpose of measurement, but membrane 34 also limitsthe velocity at which the gas stream 52 being measured flows past sensor22. Since the convective heat loss from a surface is a function of thefluid velocity of the impinging gas stream, the heat loss from sensor 22is thereby limited to a reduced amount by its location within enclosingstructure 30.

As depicted in FIG. 1, gas-permeable membrane 34 is attached at theupper end of the enclosing structure (that is, the end opposite thatmounted on the mounting surface) so as to define an interior volumewithin the enclosing structure that is isolated from the externalenvironment.

FIGS. 2 and 3 illustrate an embodiment of the present isolated gassensor configuration in assembled form (FIG. 2) and in exploded form(FIG. 3). As shown in FIGS. 2 and 3, gas sensor assembly 10 includes aflex circuit surface 15 on which are mounted a sensor 22, an enclosingstructure 30 that includes a walled component 32, and a gas-permeablemembrane 34. An interior volume or gas-filled gap is formed withinenclosing structure 30, bounded on the bottom by flex circuit 15 andsensor 22, bounded on the side by the walls of component 32, and boundedon the top by membrane 34.

As shown in FIG. 2 and better shown in FIG. 3, sensor 22 is mounted at acentral portion 15 c along the longitudinal extent of flex circuit 15.End portion 15 a of flex circuit 15 is folded under central portion 15 cto create a structure in which folded-under end portion 15 a supportscentral portion 15 c and prevents central portion 15 c from contactingthe underlying components of assembly 10. The support afforded byfolded-under end portion 15 a also enables a gap or volume to bemaintained under central portion 15 c and sensor 22 mounted thereon,thus thermally isolating sensor 22 from the underlying components ofassembly 10.

As further shown in FIGS. 2 and 3, flex circuit 15 terminates in an endportion 15 b, in which the copper traces (not shown) that extend fromsensor 22 are electrically connected to pin connectors 19. Each pinconnector has a head portion 19 a and a spiked portion 19 b. In theillustrated embodiment, a copper trace extending from gas sensor 22 iselectrically connected to pin connector head 19 a. The spiked portionsof pin connectors 19 are inserted through holes in end portion 15 b offlex circuit 15, then through aligned holes in an electricallyinsulative support layer 17. The spiked portions of pin connectors 19are insertable into aligned mounting holes in a circuit board (notshown), which contains the downstream processing and control circuitryto which the signals from sensor 22 are directed.

The gas-permeable membrane preferably has optical properties thatinhibit the passage of light into the interior volume of the enclosingstructure. The preferred gas-permeable membrane is non-transparent(preferably opaque), and is formed from a polymeric material. A suitablegas-permeable membrane material is a polytetrafluoroethylene-basedmembrane material commercially available from W.L. Gore & Associatesunder the trade name Gore™ Membrane Vents (available in various ventdiameter and airflow grades under Part Nos. VE70308, VE70510, VE70814,VE70919, VE71221 and VE72029). Other suitable non-polymericgas-permeable membrane materials could be employed as well, such as, forexample, porous metallic membranes.

In addition to reducing heat loss, the optical isolation of certain gassensors is desirable to reduce or minimize interference from light withsignals representing the gas constituent(s) being measured. The use ofcertain gas-permeable membranes can reduce or minimize the intrusion ofambient light upon gas sensor 22. Enclosing structure 30 is thereforepreferably fabricated from an opaque material so that, in conjunctionwith membrane 34, sensor 22 is kept in a light-controlled environment.

The use of a gas-permeable membrane and structural elements enclosingthe sensor reduces or minimizes the potential for mechanical intrusion.Intrusion could take the form of particles entrained in the gas beingsensed instead of the gas constituent to be detected. Such intrusionscould also take the form of accidental occurrences, such as the gassensor assembly, or the overall system in which it is integrated, beingdropped or prodded. The protection afforded by the enclosing structurereduces or minimizes the chance of damage to the sensor and itsconnections to related components.

Conventional, prior art solutions employed sintered metal disksprimarily to reduce flow sensitivity and to prevent mechanicalintrusion. These prior solutions have disadvantages in relation to thepresent gas sensor assembly, namely;

-   prior art gas sensors are significantly greater in volume than the    present solution;-   prior art gas sensors do not isolate the gas stream in which the    constituent(s) to be tested are flowed;-   prior art gas sensors form much larger isolated volumes: and-   prior art gas sensors do not inhibit light from entering the    isolated volume.

In the prior designs, trade-offs were made among the porosity of thesintered metal, its thickness, and the isolated volume. If the porosityof the sintered metal were too great (that is, having a prevalence ofopen pores in its interior volume) or if the sintered metal disk weretoo thin, the effects of flow rate were more pronounced. In this regard,low porosity of the sintered metal disk reduces response time (that is,the difference between the time a change occurs in the concentration ofthe gas constituent being detected and the time the gas sensor and itsassociated processing and control circuitry register the change).Although such sintered metal disks are generally able to block most ofthe light to which the sensor is exposed, the degree of blockage isgenerally inadequate for sensors that are sensitive to low-level light.Sensor assembly designs that employ these sintered metal isolating disksmust therefore accept trade-offs between competing performancevariables. The present design is essentially independent of such designtrade-offs, since the performance variables improve with reduced size ofthe gas sensor assembly.

In the present gas sensor assembly, the gas-permeable membrane is heldin position by walled component of the enclosing structure, which couldbe as simple as a ring formed of electrically insulative material, suchas, for example, a ring formed from an acetal resin (commerciallyavailable from DuPont under the trade name Delrin®). The enclosingstructure could also be formed in shapes, other than a ring, yet havinga hollow interior volume to accommodate the sensor. The gas-permeablemembrane is preferably placed directly over the sensor and preferablyalso encloses the wire extending between the sensor and the processorand/or other elements that make up the overall sensor assembly. Theresulting structure accommodates the use a relatively small volume of anisolated gas stream, which improves sensor response.

The isolated gas stream in the volume of the enclosing structure of thepresent gas sensor assembly also has convection properties that arereduced in relation to those of prior art sensors, in which the sensoris exposed to the sample gas volume, thereby reducing power loss in thepresent design. The performance of the present sensor assembly issubstantially unaffected by flow in the sample gas volume, since it ismechanically isolated from the sample gas volume. The barriers of theenclosing structure and gas-permeable membrane in the present gas sensorassembly also protect the sensor from various forms of intrusion andcontamination, such as from mechanical impact, droplets of liquidcontaminants, dust, and the like. Finally, the preferred membrane in thepresent gas assembly is opaque to ambient light and isolateslight-sensitive sensors from the external environment.

The advantages of the present isolated gas sensor configuration includethe isolation of the sensor from: light, sample gas stream flow,mechanical intrusion (handling, dust, and the like), and liquidcontaminant intrusion, without substantially compromising sensorresponse while providing thermal isolation.

While particular steps, elements, embodiments and applications of thepresent invention have been shown and described, it will be understood,of course, that the invention is not limited thereto since modificationscan be made by those skilled in the art, particularly in light of theforegoing teachings.

1. A gas sensor assembly for detecting a constituent in a gaseousstream, the assembly comprising: (a) a mounting surface; (b) a sensormounted on the mounting surface, said sensor capable of generating adetectable signal in the presence of said constituent; and (c) anenclosing structure comprising: (i) a walled component having a pair ofvertically spaced ends, the walled component mounted at one end on saidmounting surface and circumscribing said sensor; and (ii) agas-permeable membrane attached at the other end of said walledcomponent, thereby defining an interior volume within said enclosingstructure, said gas-permeable membrane having optical properties thatinhibit passage of light into said interior volume; whereby flowing thegaseous stream across said gas-permeable membrane infuses a portion ofthe gaseous stream into said interior volume containing said sensor,wherein said mounting surface is formed on a flexible circuit, saidflexible circuit having a central portion and two end portions, at leastone of said end portions capable of being folded under said centralportion such that said central portion does not contact underlyingcomponents of said sensor assembly.
 2. The gas sensor assembly of claim1 wherein said gas-permeable membrane is non-transparent.
 3. The gassensor assembly of claim 2 wherein said gas-permeable membrane isopaque.
 4. The gas sensor assembly of claim 1 wherein said gas-permeablemembrane is polymeric.
 5. The gas sensor assembly of claim 1 wherein thegas-permeable membrane is a polytetrafluoroethylene-based membranematerial.
 6. The gas sensor assembly of claim 1 wherein said mountingsurface is planar.
 7. The gas sensor assembly of claim 1 wherein saidsensor is sensitive to hydrogen.
 8. The gas sensor assembly of claim 7wherein said sensor is catalytically activated.
 9. The gas sensorassembly of claim 1 wherein said walled component is tubular.
 10. Thegas sensor assembly of claim 1 wherein said walled component is formedfrom an electrically insulative material.
 11. The gas sensor assembly ofclaim 10 wherein the electrically insulative material is polymeric. 12.The gas sensor assembly of claim 11 wherein the electrically insulativematerial is an acetal resin.
 13. The gas sensor assembly of claim 1wherein said at least one folded end portion forms a gap, said gapthermally isolating said sensor from said sensor assembly underlyingcomponents.
 14. A method of isolating, in a gas sensor assembly, asensor capable of generating a detectable signal in the presence of agas stream constituent, the method comprising: (a) mounting said sensoron a mounting surface and; (b) enclosing said sensor in an enclosingstructure comprising: (i) a walled component having a pair of verticallyspaced ends, the walled component mounted at one end on said mountingsurface and circumscribing said sensor; and (ii) a gas-permeablemembrane attached at the other end of said walled component, therebydefining an interior volume within said enclosing structure, saidgas-permeable membrane having optical properties that inhibit passage oflight into said interior volume; whereby flowing the gaseous streamacross said gas-permeable membrane infuses a portion of the gaseousstream into said interior volume containing said sensor, wherein saidmounting surface is formed on a flexible circuit, said flexible circuithaving a central portion and two end portions, at least one of said endportions capable of being folded under said central portion such thatsaid central portion does not contact underlying components of saidsensor assembly.
 15. The method of claim 14 wherein said gas-permeablemembrane is non-transparent.
 16. A method of detecting a constituent ina gaseous stream, the method comprising: (a) mounting a sensor on amounting surface, said sensor capable of generating a detectable signalin the presence of said constituent; (b) enclosing said sensor in anenclosing structure comprising: (i) a walled component having a pair ofvertically spaced ends, the walled component mounted at one end on saidmounting surface and circumscribing said sensor, wherein said mountingsurface is formed on a flexible circuit, said flexible circuit having acentral portion and two end portions, at least one of said end portionscapable of being folded under said central portion such that saidcentral portion does not contact underlying components of said sensorassembly; and (ii) a gas-permeable membrane attached at the other end ofsaid walled component, thereby defining an interior volume within saidenclosing structure, wherein said gas-permeable membrane has opticalproperties that inhibit the passage of light into said interior volume;(d) flowing the gaseous stream across said gas-permeable membranewhereby a portion of said gaseous stream infuses into said interiorvolume and the impinges upon said sensor; and (e) generating adetectable signal from said sensor in the presence of said constituent.17. The method of claim 16 wherein said gas-permeable membrane isnon-transparent.